fftools/ffmpeg_opt: validate stream index in negative map handling

Negative -map processing iterates previously parsed stream map entries
and dereferences input_files[m->file_index]->ctx->streams[m->stream_index]
without validating that stream_index is in range.

A malformed earlier map can leave m->stream_index negative, which causes
an out-of-bounds read when a later negative map walks existing entries.
Check that stream_index is non-negative and below nb_streams before
calling stream_specifier_match().

*Vulnerability reported by Zhenpeng (Leo) Lin at depthfirst*
*Patch validated by Zheng Yu at depthfirst*

Fixes: DFVULN-695

fftools/ffmpeg_opt.c

@@ -645,6 +645,8 @@ static int opt_map(void *optctx, const char *opt, const char *arg)
             for (i = 0; i < o->nb_stream_maps; i++) {
                 m = &o->stream_maps[i];
                 if (file_idx == m->file_index &&
+                    m->stream_index >= 0 &&
+                    m->stream_index < input_files[m->file_index]->nb_streams &&
                     stream_specifier_match(&ss,
                                            input_files[m->file_index]->ctx,
                                            input_files[m->file_index]->ctx->streams[m->stream_index],